so i hooked one of those AliExpress "HDMI Video Capture" dongles up to my Linux machine's HDMI output, which actually turned out to work this time. you just have to make sure it's connected when the machine boots, so it knows which video output to send BIOS and console to.
also, it turns out that the preview window in QuickTime Player's built-in Movie Capture mode is a lot less laggy than OBS. i was able to use it as a virtual monitor and type on a keyboard that was attached to the Linux machine.
here's a capture from the dongle: